project cannot save this project until all required fields have valid values [Microsoft Project 2007]

Did you see the notification above when you save your project to the Project server ? .
So you can do this step :
1. Check the required fields that you have to fill. (Project/Project Information)
    you can see the required fields noted with the asterisk symbol.
2. If you have fill all of the required fields, so we could assume that something wrong even bug with the Microsoft project Client. You can see the hotfix on microsoft support